docs: def from_json(cls, string):
          """Create a ChemicalEntity from a json string 
          """
          exp_dict = json_to_data(string)
          version = exp_dict.get('version', 0)
          if version == 0:
              return cls.from_dict(exp_dict)
          elif version == 1:
              return cls.from_dict(exp_dict)
          else:
              raise ValueError("Version %d not supported" % version)